The present invention relates to a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a mask table provided with a mask holder for holding a mask;
a substrate table provided with a substrate holder for holding a substrate;
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
More specifically, the invention relates to subsystems of such an apparatus which employ coherent light beams, e.g. as produced by a laser. In particular, the invention relates to an interferometric measurement system, especially as employed in an alignment system, a leveling system or a stage-position-measuring system, for example.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies that are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commnonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed xcexd at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97133205.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine.
The projection radiation in current lithographic devices is generally UV (ultra-violet) light with a wavelength of 365 nm, 248 nm or 193 nm. However, the continual shrinkage of design rules in the semiconductor industry is leading to an increasing demand for new radiation types. Current candidates for the near future include UV light with wavelengths of 157 nm or 126 nm, as well as extreme UV light (EUV) and particle beams; (e.g. electron or ion beams).
Interferometric measurement systems can be employed in a variety of subsystems in a lithographic projection apparatus, for example:
for the precision alignment of a mask relative to a substrate;
for the precise leveling of a substrate and/or mask relative to the projection beam, as directed onto a particular target area of the substrate;
for the accurate determination of position, velocity and/or acceleration of the substrate and/or mask stages.
In one example of an interferometric measurement system, a coherent light beam is divided into two beams, for example by a diffraction grating provided on a first component (e.g. a reticle). The two beams are then recombined to form an interference pattern that impinges on a transmission grating provided on a second component (e.g. a wafer). The coincidence of the interference pattern and the second grating produces a Moire pattern. The total transmitted intensity varies with the phase of the Moire pattern, which is in turn a function of the relative displacement of the first and second components. Therefore, the relative positions of the components can be determined with high resolution by detecting the overall transmitted intensity.
It should be explicitly noted that the projection radiation used in the lithographic projection apparatus may be of a different type or wavelength to the coherent light beam used in an interferometric measurement system as hereabove described. For example, in the case of a DUV projection apparatus, the projection wavelength is 248 nm, but the alignment system, leveling system and stage-position-measurement system often employ radiation from a HeNe laser (wavelength=632 nm), or a diode laser, for example.
In an interferometric measurement system as here described, it is important that the positions of the said first and second components can be determined reproducibly (e.g. so that registration marks are correctly aligned for different exposures). However, considerable problems can be caused by the presence of noise in the measurement signal produced by the system, which can lead to poor reproducibility and reduced measurement precision (in turn leading to significant errors in such factors as overlay performance, focus, and scan synchronization, for example).
It is an object of the invention to alleviate this problem.
Accordingly, the presently invention provides an optical measurement system comprising:
a source of substantially coherent radiation;
a detector for detecting a desired signal resulting from interference between beams derived from said source,
characterized by a phase modulator for modulating the phase of radiation emitted by said radiation source, whereby detection of at least one spurious signal, caused by interference from at least one spurious beam in the optical measurement system, is suppressed.
In experiments leading to the invention, the present inventors determined that noise in the measurement signal of such an optical measurement system can be caused by the presence of a spurious radiation beam, e.g. arising from:
an unmasked reflection at an air/glass interface within the (typically) complex projection lens of the lithographic projection apparatus, or within other optical systems, e.g. as used in the radiation system or alignment system;
a similar reflection occurring at the reticle.
However, the precise location of such a reflection is generally difficult to determine. Moreover, the provision of anti-reflection coatings on the optical components of the apparatus does not generally provide an effective cure of this problem.
Advantageously, the present invention enables the noise signal to be substantially eliminated without any modification to the detection system: it is only necessary to provide a phase modulator in the radiation source beam path.
Preferably, the phase modulator comprises a controllable attenuator for sequentially modulating the radiation at a plurality of modulation amplitudes. This has the advantage of enabling additional spurious signals to be suppressed, and of allowing suppression of noise signals caused by spurious reflections at a range of different positions within the optical measurement system.
Preferably the phase modulator comprises an electro-optic element, such as an LiNbO3 element, for example. A modulator as here discussed is described, for example, in U.S. Pat. No. 5,189,547, which is incorporated herein by reference.
The above aspects of the invention will be explained thoroughly in the Embodiments below.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.